1. Field of the Invention
This invention relates to luminescent ceramics, such as those used in light-emitting devices.
2. Description of the Related Art
White light-emitting diodes (LED) are well-known solid state lighting devices and have been widely put to practical use. Examples of uses of LEDs include indicators for various instruments, backlighting for LCD displays used in cellular phones, signboards, ornamental illumination, etc. For some applications, it is difficult to obtain an LED which emits light in the color range desired for the application. For example, many LEDs emit blue light, but often white light is desired for a device. In these situations, phosphors can be used to change the color of the emitted light. This is done by allowing blue or some other colored light emitted from the LED to pass through the phosphor. Some of the light passes through the phosphor unaltered, but some of the light is absorbed by the phosphor, which then emits light of a different wavelength. Thus, the phosphor tunes the apparent color of the emitted light by converting part of the light to light of a different wavelength. Many white light-emitting devices are based upon this type of color conversion. For example, one type of conventional white-light emitting device comprises a blue-LED and yellow light emitting YAG phosphor powder dispersed in encapsulant resin such as epoxy or silicone.
The luminous efficiency of LEDs has improved in recent years. As a result, LED use may expand to white light-emitting devices which require more luminance intensity such as headlights for automobiles, backlighting for mid- to large-size LCD displays, and general lighting replacing current fluorescent and incandescent lamps. For these applications, it may be helpful have a light-emitting device that maintains its emission efficiency under higher driving conditions and greater emission flux. In some instances, higher driving conditions may greatly increase heat generation in the LED device, and this increased temperature may reduce both LED efficiency and device lifetime. For example, a temperature rise may cause a reduction of the internal quantum efficiency of an LED semiconductor chip and a shortening of the encapsulant resin's lifetime. Recently, LED devices have been prepared which use a luminescent ceramic plate instead of a powder. This has helped reduce thermal quenching somewhat, possibly due to the better thermal conductivity of the plate as compared to a powder dispersed in a resin. However, thermal quenching remains a problem even for ceramic plates.